Conventionally, a protection element which is mounted on a secondary battery device or the like is employed as the one that has a function of preventing an overvoltage as well as an overcurrent. This protection element is formed so that: a heating element and a soluble conductor made of a low-melting metal member are laminated on a board; the soluble conductor is blown out due to an overcurrent; and in case that an overvoltage is generated as well, power is supplied to the heating element in the protection element and then the soluble conductor is blown out due to a heat of the heating element. Blowout of the soluble conductor takes place due to goodness of wettability relative to a connected electrode surface at the time of blowout of the soluble conductor that is a low-melting metal. The low-melting metal that has been blown out is attracted onto an electrode, and as a result, the soluble conductor is broken and then a current is shut off.
On the other hand, with downsizing of an electronic device, such as a portable device, in recent years, a protection element of this type has been needed to be downsized or reduced in thickness; and there has been a further demand for operational stability and fastness. As a means therefor, there is provided the one in which a soluble conductor of a low-melting metal member is disposed on an insulation board; the disposed soluble conductor is sealed with an insulation cover; and a flux is coated onto the soluble conductor. This flux is adapted to prevent oxidization of a surface of the soluble conductor, and is provided so that the soluble conductor blows out speedily and stably at the time of heating the soluble conductor.
Such a protection element has a structure shown in FIG. 9. In this protection element, a pair of electrodes 2 is provided on a baseboard 1, and a pair of electrodes, although not shown, is provided at an opposite edge part which is orthogonal to the electrodes 2. A heating element 5 made of a resistor is provided between electrodes, although not shown, and a conductor layer 7 which is connected to one of a pair of electrodes, although not shown, via an insulation layer 6, is provided. At this protection element, a soluble conductor 3 made of a low-melting metal foil is provided between a pair of electrodes 2 that is formed on both ends of the baseboard 1. A center part of the soluble conductor 3 is provided on the conductor layer 7. Further, an insulation cover 4 is provided in face-to-face opposite to the soluble conductor 3 that is provided on the baseboard 1. The insulation cover 4 which is mounted on the baseboard 1 is put with a predetermined space 8 being formed relative to the soluble conductor 3. A flux 9 is applied to the soluble conductor 3, and the flux 9 is housed in the space 8 which is provided in the insulation cover 4.
In addition, a protection element having a soluble conductor which is sealed with an insulation cover has a structure disclosed in Patent Document 1. In this protection element, a space in which a fused metal gathers on an element at the time of blowout of the soluble conductor is small due to reduction in thickness, and thus, in order to ensure drawing of the fused metal into each electrode portion, a metal pattern with its good wettability relative to the fused metal is provided at a site which is face-to-face opposite to each electrode on an interior face of the insulation cover so that the fused metal is speedily drawn into each electrode forming portion.
Moreover, as disclosed in Patent Document 2, there is proposed the one in which: a flux is coated onto a soluble alloy piece in order to prevent a difference in operation temperature; and a belt member of groove or glass for preventing wetting and spreading of a fused alloy is provided at the periphery of an electrode to which a soluble alloy is connected.